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Topic: Educate me about why 70mm <> 70mm... (Read 7776 times)

Alrighty, I should probably know this, but I guess I've never had the lenses to test this question before.

Yesterday evening, I received a copy of the new Canon 24-70 F4 IS lens. We can talk about why I selected that lens over the F2.8 and why I'm so crazy, but perhaps folks can explain to me something I observed this morning on my morning walkabout where I took the new lens out for a shakedown cruise.

The images below are uncropped and unretouched, other than being exported from Lightroom at smaller dimensions than the original RAW files from my 5d Mk II.

The first image (Img_4708.jpg) is the 24-70 shot at 70mm (at the top of the lens's rotational range) F4, 1/200th, 1000 ISO, -1/3 EV.The second image (Img_4697.jpg) is the 70-200 Mk II shot at 70mm (at the bottom of the lens's rotational range) F4, 1/160th, 1000 ISO, -1/3 EV

Both images were handheld (left my tripod at home this morning), but I didn't move between switching lenses. Looking at the LCD, I thought "wait, is one of those more zoomed-in than the other?"

At home, thinking perhaps it was how I'd held the camera, I set up the tripod in the kitchen and repeated the test. This time, the tripod definitely didn't move between images.

As you can see, in both sets, there's a difference in the angle of view - not necessarily good or bag, but different. So, I can come up with two explanations for what I'm seeing, but I'm hoping someone can shed more light on this.

1. The difference can be explained by the fact that lens manufacturers paint numbers on the lenses which are approximations of the focal length, and two lenses of different design will inevitably have a different length (or angle of view) even at what's nominally the same setting.

or

2. This is the optical result of the difference in physical length between the 24-70 and the 70-200 lenses. At 70mm, the 24-70 is about 3 inches shorter than the 70-200. Because in both tests I tried to keep the focal plane of the image sensor more or less in the same place (roughly in the handheld setting, exactly when it was on the tripod), the front element of the 70-200 was always closer to the subjects, thereby shifting the scene.

My question to the collective wisdom of folks: Are either or both of these explanations accurate, or is there something else at play here? And also, what might explain the approximately 1/3 stop difference that the camera's choosing between shutter speed when it's working with the different lenses? Is this also due to the front element being closer to the subject (that whole "light falling off at the square of its distance" thingy I learned and forgot in my college photography class so long ago?)

canon rumors FORUM

It could be the reasons you listed, but it could also be the fact that the same distance from the subject is not at the same spot on the lenses' focussing distances. On the 24-70 it might be at halfway between macro and infinity but on the 70-200 it will surely be closer to its closest focussing distance. And because of focus breathing, the angle of view changes. I don't know how much either of the lenses breathe, but the Nikon 70-200 gives an angle of 135mm at 200mm and closest focussing so there's lots of room for imprecision.

1) All lenses are measured at infinity focus; most of what we call zoom lenses suffer from focus breathing, whereby the focal length changes as you change the focus: internally zooming lenses (i.e those with rear focus groups) are the worst culprits (actually, true zooms are parfocal). Try repeating your experiment with each lens focussed at infinity.

2) The focal length quoted is approximate and can vary between lens models; Canon's website should be able to give you the exact technical specifications if you're interested (sorry, I'm being lazy and can't be bothered to look them up!).

In both instances, the image on the lower pic is larger. Did you measure from the focal plane of the camera?

+1 Notice the focal plane marking on the top of your camera. Try a more precise test measuring from that point.

I think you have an excellent question. What follows is my opinion and I also look forward to a more educated answer.

Refer back to your option #1. The numbers painted on the lens are indeed somewhat approximated. Like all other things in the real world, nothing is totally exact. There are always slight variations to the established range because the range designation is meant for reference, not perfection. It should be somewhat identical to all other lenses of that design, but not exactly the same when compared to other designs. You will find that the aperture is also not exact when compared to the spec, if you could measure it exactly that is. This variation from spec is seen in many reviews (esp when super-tele-zooms are reviewed) where the advertised apertures and zoom ranges are tested. The advertised numbers are often a bit inflated to make the lens seem a bit better than it really is.

I don't think there is anything wrong, it's just a good observation on your part. I look forward to the discussion...

Logged

Yes, but what would surapon say ??

canon rumors FORUM

I have noticed by looking at the patents that we see detailed here on Canonrumors.com sometimes that the REAL focal length and the REAL aperture are not the same as the MARKED focal length and MARKED aperture.

What I mean is you see a patent for a 50mm f 1.4 lens and the details on the patent will say something like it's really a 51.2 mm and f 1.51. (not real numbers here, just an example I made up)

So maybe "70" on the 24-70 is really 68.2mm and "70" on the 70-200 is 73.1mm.

Don't know this for sure, just something I have seen from the patents and am speculating on.

In both instances, the image on the lower pic is larger. Did you measure from the focal plane of the camera?

This assumes some how that 70mm or any other focal length *must* have a certain framing...which may not be the case. The focal length designated might be just tied to the optical system...lets not forget these are multi lens designs. So it may be perfectly ok to have slightly different FOV for a given focal length depending on the optical system.

As an anology, i am guessing (not certain) that if one takes two refracting telescopes of exact same focal length but one scope has a larger objective say 100mm vs the other's 30 mm, in addition to their vastly different light gathering power, they may also frame the sky more or less, even though they have the same FL.

lodgepole

A bit off topic, but I'm also very curious about the apparent large differences in the depth of field of the two lenses. In both images, the 24-70 f4 seems to have much more DOF. Is this common between two lenses at the same focal length and aperture?

A bit off topic, but I'm also very curious about the apparent large differences in the depth of field of the two lenses. In both images, the 24-70 f4 seems to have much more DOF. Is this common between two lenses at the same focal length and aperture?

In part, it's due to the FoV seeming to be a bit less on the 70-200, but also very likely due to f/2.8 vs f/4. The larger the aperture (smaller f-number), generally the shallower DoF. Also influencing that is distance from lens to subject, distance between subject and background/foreground.

... imho in this case it's a rather severe difference, because a) given the short zoom range a few mm more or less do make a difference, esp. a difference on the wide end matters and b) the sharpness is always compared the "wide end", but in reality it should be like 24mm mk2 vs 26mm mk1 which might make some sharpness difference.